The below is from the Netscape 6.2 Help. I believe you need to specify which application (Java Applet Launcher) is to handle Java file types (.jar for one, but I belive there are more). Deep breath:
Managing Different File Types
Navigator can handle many types of files. However, for some files, such as movies, Navigator launches a "helper" application that can handle the file, or it saves the file for you to view later.
The settings in the Helper Applications preferences panel tell Navigator which helper applications to use on your computer. You probably don't need to change them because they come preconfigured. Also, if Navigator doesn't know which helper application to use, it allows you to find helper applications within the download dialog box.
The download dialog box has these options:
* Open using: Netsscape opens the file using the default program in your system settings.
* Choose: Netscape allow you to open the file using an application you choose.
* Save this file to Disk: Netscape allows you to select a location to save the file.
* Always ask before opening this type of file: When this is selected, Netscape opens the download dialog box when downloading a particular file type. You can reset these choices afterwards in preferences for Helper Applications.
* Advanced: Netscape allows you to fine-tune how files are handled. See "To add a new helper application to the list" for details.
For experienced users to fine-tune the Helper Applications preferences:
1. Open the Edit menu and choose Preferences.
2. Under the Navigator category, click Helper Applications. (If no options are visible, click to expand the list.)
3. Perform any of the following procedures:
To add a new helper application to the list:
1. Click New Type. You see the New type dialog box.
2. Type a description of the type of files the application works with; for instance, "movies" or "sound files."
3. Type the file extension (suffix) used by this file type. If this type of file uses more than one extension, type them all, separated by spaces.
4. Type the MIME type. MIME types consist of two words separated by a slash. For instance, a TIFF image file's MIME type is "image/tiff".
Note: When you download a file, Netscape will show you the MIME type on the download dialog box.
5. Click Choose, then select an application that handles files of this type.
6. Click OK.
To specify how Navigator should handle downloaded files:
1. Select a file type from the list and click Edit.
2. Choose a "Handled By" option to have downloaded files saved on your hard disk or opened in an application that you specify. Click Choose to specify the application.
3. Select "Ask me before opening downloaded files of this type" if you want to be notified before downloading files that are handled by applications other than Navigator.
4. Click OK.
To remove an item from the list:
1. Select a file type from the list and click Remove.
2. Click OK.